Methods and apparatus to control charge neutralization reactions in ion traps

Abstract

An ion trap mass spectrometer uses electrospray ionization to introduce multiply-charged positive ions in an axial direction into a quadrupole ion trap and glow discharge ionization to introduce singly-charged negative ions in a radial direction into the ion trap. Methods of controlling ion-to-ion charge transfer reactions include applying dipolar DC voltage across endcap electrodes to allow partial charge state neutralization reactions to occur between the positive and negative ions and then suspending further charge state neutralization reactions. The remaining ions can be further processed and transformed and a mass spectrum created by scanning a quadrupolar RF field.

What is claimed is:  
     
       1. A method of controlling an ion trap comprising the steps of 
       generating an RF field for simultaneous trapping of positive ions and negative ions,  
       trapping first ions having charge states of one polarity by the RF field,  
       trapping second ions having charge states of opposite polarity by the RF field,  
       at least one of the first ions and second ions being in a multiply-charged state,  
       applying a DC field to spatially separate the first ions and second ions to permit charge state neutralization reactions to partially migrate the multiply-charged state to at least one target lesser charge state having a higher mass-to-charge ratio.  
     
     
       2. The method of  claim 1  in which the DC field has an amplitude adjustable by an operator to select different target charge states having different mass-to-charge ratios. 
     
     
       3. The method of  claim 1  in which a low mass cutoff of the ion trap is altered by an operator to select different target charge states. 
     
     
       4. The method of  claim 1  in which the DC field has a dipolar DC component to inhibit further charge state neutralization reactions upon migration to the target lesser charge state. 
     
     
       5. The method of  claim 4  in which the ion trap has at least a pair of spaced electrodes, and the dipolar DC component is created by applying a first DC voltage of an adjustable amplitude to one of the electrodes and applying a second DC voltage of different but adjustable amplitude to the other of the electrodes. 
     
     
       6. The method of  claim 5  in which the first DC voltage and second DC voltage are of equal magnitude but opposite polarity and are adjustable by an operator to alter an end point of migration. 
     
     
       7. The method of  claim 6  in which the ion trap has a pair of endcap electrodes corresponding to the pair of spaced electrodes and has a ring electrode, and the dipolar component is created by applying a positive polarity DC voltage to one endcap electrode and a negative polarity DC voltage to the other endcap electrode with the positive and negative polarity DC voltages having the same absolute magnitude. 
     
     
       8. The method of  claim 1  in which an operator can vary at least a time period of applying the DC field to vary the target lesser charge state. 
     
     
       9. The method of  claim 1  including establishing an accumulation time period for accumulating of first ions and second ions and a separate time period for applying the DC field which overlaps at least a portion of and extends beyond the accumulation time period. 
     
     
       10. The method of  claim 1  in which trapping first ions and trapping second ions occurs sequentially in time, suppressing the DC field during trapping first ions, and applying the DC field at least during trapping second ions. 
     
     
       11. The method of  claim 1  in which the ion trap has a pair of endcap electrodes having an axial opening and a ring electrode having a radial opening, the trapping first ions occurs by introducing first ions through one of the axial opening and radial opening, and the trapping second ions occurs by introducing second ions through the other of the axial opening and radial opening. 
     
     
       12. The method of  claim 11  in which ions introduced through the axial opening occurs during a first period of accumulation and ions introduced through the radial opening occurs during a separate period of accumulation. 
     
     
       13. The method of  claim 1  in which the at least one of the first ions and second ions have a plurality of different multiply-charged states to create a distribution of multiply-charged states. 
     
     
       14. The method of  claim 13  in which at least one of the first ions and second ions are created by electrospray ionization of an analyte sample to thereby produce a broad distribution of higher multiply-charged states. 
     
     
       15. The method of  claim 1  in which another of the first ions and second ions are created by glow discharge ionization of a reagent to produce at least a singly charged state. 
     
     
       16. The method of  claim 15  including generating a DC gating signal applied to an electrode downstream of the glow discharge ionization to pass the singly charged ions during an accumulation period and to thereafter block passage of the singly charged ions. 
     
     
       17. The method of  claim 1  including quenching of one of the first ions and second ions after applying the DC field to prevent further charge state neutralization reactions to thereby maintain the target lesser charge state. 
     
     
       18. The method of  claim 17  in which quenching includes continuing the DC field and adjusting the RF field to eliminate the one of the first ions and second ions. 
     
     
       19. A method of controlling an ion trap comprising the steps of 
       generating an RF field for simultaneous trapping of positive ions and negative ions,  
       accumulating first ions having charge states of one polarity within the RF field during a first time period,  
       accumulating second ions having charge states of opposite polarity within the RF field during a second time period,  
       one of the first ions and second ions being in a multiply-charged state and the other of the first ions and second ions being in at least a singly charged state,  
       applying a DC field for a third time period which extends beyond the second time period to disperse the first ions and second ions within the RF field to partially permit and then suspend charge state neutralization reactions to thereby migrate the multiply-charged state to lesser charge states having higher mass-to-charge ratios, and  
       varying at least one of the third time period, the RF field, and the DC field to select a particular one of the lesser charge states as a target charge state.  
     
     
       20. The method of  claim 19  in which the step of varying includes allowing an operator to select an adjustable amplitude for the DC field as one parameter to select the target charge state. 
     
     
       21. The method of  claim 19  in which the step of varying includes allowing an operator to select adjustable durations for the third time period and at least one of the first and second time periods as parameters to select the target charge state. 
     
     
       22. The method of  claim 19  including expulsion of the other ions having at least the singly charged state during a fourth time period following the third time period to quench the reactions and thereby maintain the target charge state. 
     
     
       23. The method of  claim 22  in which expulsion occurs by adjusting the low mass-to-charge cut-off for the RF field to eliminate the singly charged ions. 
     
     
       24. The method of  claim 19  in which the ion trap has at least a pair of spaced electrodes, and the DC field is created by applying a positive polarity DC voltage of an adjustable amplitude to one of the electrodes and applying a negative polarity DC voltage of an adjustable amplitude to the other of the electrodes. 
     
     
       25. The method of  claim 19  in which the third time period begins during the first time period or second time period so that the DC field overlaps accumulating of at least certain ions and extends until at least suspension of the charge state neutralization reactions.